The present invention relates to fiber amplifiers having filter means for attenuating or removing certain specified wavelengths, and to resonant ring fiber filters for use in such amplifiers.
Doped optical fiber amplifiers consist of a gain fiber the core of which contains a dopant such as rare earth ions. The gain fiber receives an optical signal of wavelength .lambda..sub.s and a pump signal of wavelength .lambda..sub.p which are combined by means such as one or more couplers located at one or both ends of the gain fiber. The spectral gain of a fiber amplifier is not uniform through the entire emission band. For example, an erbium doped gain fiber, the gain band of which coincides with the 1550 nm telecommunications window of silica based optical fiber, has an irregular gain spectrum that includes a narrow peak around 1536 nm. Fiber amplifier gain spectrum modification has been employed in fiber amplifiers for such purposes as gain flattening and gain narrowing.
It is known that a gain fiber can include a distributed filter for improving the efficiency of a fiber amplifer and/or tailoring the spectral output thereof. Such a distributed filter/gain fiber has an active ion-doped core that is located along the fiber axis, and it further includes a second, off-axis core that extends parallel to the active ion-doped core. The two cores have different characteristics such as core diameters and/or refractive index profiles. The structure can support at least two core modes, and the propagation constants of the two core modes can be manipulated independently by proper selection of the aforementioned characteristics. The cores can therefore be designed such that their propagation constants are equal at a certain resonant wavelength, .lambda..sub.0. At wavelength .lambda..sub.0 the fundamental mode of the structure changes from one core to another. Strong power transfer between the two cores can happen only at a narrow band of wavelengths centered about the resonant wavelength. If the second core contains a light absorbing material, it will absorb at least a portion of the light centered about wavelength .lambda..sub.0 to provide a filtering function that modifies the fiber amplifier gain spectrum.
It is difficult to make a fiber having two parallel cores because of its lack of circular symmetry. Also, a filter having two parallel cores is polarization dependent.
These disadvantages could be avoided by providing the amplifier with a known coaxial coupler of the type wherein a ring core is concentric with and radially spaced from the central active core to form a device referred to herein as a resonant ring fiber (RRF). At least two modes exist in a RRF. Any mode with most of its power in the core is defined as a core mode, and any mode with most of its power in the ring is defined as a ring mode. The propagation constants of one of the core modes and one of the ring modes of a RRF can be manipulated independently by varying the parameters of the core and ring. The two modes of the RRF structure behave in the same way as the two modes in the parallel core fiber coupler/filter described above, but the RRF is much easier to make using vapor deposition-based conventional fiber fabrication technology; moreover, it is intrinsically not polarization dependent due to its circular symmetry.
In the aforementioned parallel core coupler/filter, differing amounts of power can be attenuated in the off-axis core, depending upon the concentration of light absorbing dopant material contained in that off-axis core. After the parallel core filter is made, the amount of attenuation per unit length therein is fixed. If manufacturing tolerances were such that a predetermined length of fiber did not provide the desired attenuation, it would be desirable to be able to tune the attenuation to the desired value.